1. Field of the Invention
This invention is related to a digital noise reduction device for digital image processing, and actually intends to provide a noise-reduction filter as preprocessing with interframe coding based digital video codes for better image fidelity and quality.
2. Prior Art
First Prior Art
FIG. 7 is a block diagram showing an example of the refilter wellknown. A subtracter 2 subtracts an previous image signal S5, which is stored in the frame memory 5, from an original image input signal S1, which is input from the input port 1, pixel by pixel. As the result, a difference signal S2 is outputted from the subtracter 2, and supplied to a non-linear circuit 3. The difference signal S2 is transformed according to characteristics of the non-linear circuit 3.
Transformed difference value S3 is supplied to an adder 4, wherein the difference value S3 is added with the previous frame signal S5 which designates a previous frame image from the frame memory 5. The adder 4 outputs a result of addition as an additional signal S4, then this additional signal S4 is outputted from the output port 6, and is also stored into the frame memory 5.
In the above-mentioned operation, for example, the non-linear circuit 3 has characteristics which suppress the amplitude of an input signal by non-linearzation when the absolute value of the amplitude of the input signal is smaller than the preset value V1 as shown FIG. 8. In other words, the input signal S1 includes noise, such as Flicker noise related to fluorescent lights or camera noise, which has generally smaller amplitude than an interframe difference signal. So that, smaller noise signals than the preset value V1 in terms of the absolute amplitude are suppressed through the non-linear circuit 3 as shown in FIG. 8. Therefore, the preset value V1 is usually preset at almost the maximum value of included noise in the input signal S1.
Behavior of noise suppression by the non-linear circuit 3 will be explained by using FIG. 9. In FIG. 9, the solid line expresses the output signal S2 of the subtracter 2 in FIG. 7, which includes the flicker noise ingredient denoted by the dashed line. And, reference A denotes the period of random noise, and B denotes the period of motion of objects, respectively. If the preset value V1 in the non-linear circuit 3 is preset at the larger value than the maximum value like the dashed dotted line, all signals whose absolute amplitude are smaller than V1 are suppressed as noise, including effective signals during B1 period.
Second Prior Art
Pre-filtering methods, that includes motive/static discrimination about the image, have been proposed. An example of them is the method which decides the noise reduction characteristics after discrimination of not only the interframe differences but also neighbor pixels differences for motive/static evaluation. (Japanese Patent Laid-Open Publication No. 1-143583).
In this methods, the interframe absolute differences are used for motive/static discrimination, which consists of the target pixel's interframe absolute difference and the summation of the neighbor pixel's interframe absolute differences. Noise reduction characteristics changes continuously corresponding with the result of discrimination for each pixel.
As another example, the center pixel is pointed to be noise-reduced at first. All interframe absolute differences near the center pixel are compared with given constants, and motive/static discrimination is made based on these comparison. Then if discrimination designates static conditions, noise-reduction value will be increased. However, If discrimination designates motive conditions, noise-reduction value will be decreased. (Japanese Patent Laid-Open Publication No. 2-7773).
Third Prior Art
Next, FIG. 10 is a block diagram showing the construction of an another Prior Art. In FIG. 10, a digitized input image signal S6 is supplied to a subtracter 8 through an input port 7. At this subtracter 8, a frame memory data S10 is supplied from the frame memory 11. So that, an interframe difference S7 is obtained at the subtracter 8 as difference between a previous image and an original input signal. An interframe difference signal S7 is supplied to a multiplier 9, and is multiplied by multiplication coefficient K to obtain a multiplied signal S8. Then the multiplied signal S8 and the frame memory output signal S10 are supplied to the adder 10, and are added. The adder 10 is outputted a output image signal S9 to an output port 12 and the frame memory 11.
In the frame memory 11, one frame of image data is stored. Generally, the multiplication coefficient K at the multiplier 9 is 1 or less, and is usually applied to every pixels by means of some motion detector (not described in the figure) which detects a motion of the image. The coefficient K for the each pixel is different value, and is multiplied in the multiplier 9.
According to the noise reduction device described in the above, in FIG. 7, in the case that the amplitude of an interframe difference signal S2 is smaller than the preset value V1, this signal S2 is suppressed as noise. Therefore this preset value V1 should be as small as possible in order to reconstruct high fidelity of image.
However, in the cases of video-conferences and video-phones as in-house use, most of lights are fluorescent ones, and cheap TV cameras are usually utilized, so that the level of noise which is caused by above-mentioned circumstances, is relatively large. Thus, the preset value V1 has to be set as relatively larger. Consequently, if the difference signal S2 is a normal signal but not noise, such as the signal of period B1 in FIG. 9, is suppressed as noise.
As explained above, in this first prior art using the prefilter technique in FIG. 7, the signal of motion part which has the small difference value of the motion image signal, is not reconstructed with high fidelity, thus the image becomes unnatural, because the effective signals are sometimes suppressed.
According to two examples of the motive/static discrimination techniques described before, these techniques can obtain better results than the motive/static evaluation based on each pixel's interframe difference only. Because the possibility of descrimination-mistakes, in which larger amplitude signals in the static area are discriminated as signals in the motive area, decreases.
However, even if these techniques are adopted, relatively large noises sometimes make the motive/static discrimination mistaken for the supplied image signal. In the first case, for example, when the motive/static evaluation value, X.sub.s1 =.vertline.X.sub.o .vertline.+.vertline.X.sub.1 .vertline.+.vertline.X.sub.2 .vertline.+.vertline.X.sub.3 .vertline.+ . . . +.vertline.Xn.vertline. is smaller than the preset threshold or the adaptive threshold changing according to .vertline.X.sub.o .vertline. and X.sub.s1, the target pixel is discriminated as a pixel in the static area, where X.sub.o is the target pixel's interframe difference, and X.sub.1, X.sub.2, . . . Xn are neighbor pixels' interframe differences, respectively.
However, noise which includes higher frequency spectrum makes these difference values X.sub.1, X.sub.2, X.sub.3, . . . , Xn, both positive and negative. Therefore, if absolute summation of them are used for motive/static evaluation, the threshold must be set relatively larger. Consequently a motive signal with a small interframe difference could be discriminated as a signal in the static area, and be redacted as noise, because of too strong noise reduction effect.
As the result, even effective motive signals sometimes suffer from distortion, which brings unnatural artifact such as residual images.
On the other hand, in the second case, when all absolute of interframe differences X.sub.1,X.sub.2,X.sub.3, . . . Xn of each neighbor pixel, are smaller than the preset threshold, that is, X.sub.s2 =max(.vertline.X.sub.1 .vertline.,.vertline.X.sub.2 .vertline., . . . ,.vertline.Xn.vertline.).times.n is smaller than the preset threshold, the center point of the target area is discriminated as the pixel in the static area. Therefore, it is necessary for this preset threshold to be set relatively larger in order to suppress higher frequency noise. Consequently this second case has also the same problem as first case.
In the case of the third prior art showing FIG. 10, when the coefficient K is exactly the same as 1, the interframe difference S7 is directly sent to the output port 12 as an output image signal S9. If the input image signal S6 includes noise at this time, the noise is also looked as a part of interframe difference S7 from the stored data in the frame memory 11, and is supplied to the multiplier 9. At the multiplier 9, the noise is multiplied by the coefficient K(=1), then it directly appears at the output port 12. In this case, noise reduction cannot be provided.
When the coefficient K is smaller than 1, a difference in an input image signal S6 appears as the value multiplied by K in the output image signal S9, so that this difference in the input image signal S6 affects the output image signal S9 several frames later only to reconstruct the image with some fidelity.
Therefore the smaller the coefficient K is, the more a random type of signal such as noise is suppressed as noise reduction effect. However even effective difference from the previous frame appears to be reconstructed several frames later with some fidelity, which makes the distortion that trails a tail in the image, called "comet tail". Then the multiplier 9 is operated in order that the coefficient K is set smaller than 1 for almost static image to perform noise reduction effect, and is set 1 or close to 1 for motive image.
In other words, the coefficient K is set only by motion independent of the image brightness.
As the result, when the difference from the previous frame is relatively large, noise reduction effect decrease because the coefficient K is set a value close to 1. On the other hand, the distortion called "comet tail" conspicuously appears according to increase of noise reduction effect.